As a preliminary matter, wood can be viewed as consisting of two major components, carbohydrates and lignin. Other components constitute a minor part of the wood and manifest as intercellular material, and extraneous substances that are related to the growth of the cells of the tree. The cell walls of the wood are composed of polysaccharides, the chief of which is cellulose. Lignin, on the other hand, is an amorphous substance, partly aromatic in nature, that has been called a "cementing material" or an "encrusting substance." It is insoluble in water and in most common organic solvents. It is also insoluble in acids, but undergoes condensation reactions in the presence of strong mineral acids. Lignin is partly soluble in alkaline solutions and is readily attacked and solubilized by oxidizing agents.
The extraneous substances of wood are deposited as cells grow, or after they reach maturity. Most of these substances are relatively simple compounds, having a low molecular weight. These low molecular weight substances include pectins, proteins, and like substances that are soluble in water or neutral organic solvents. The extraneous substances also include "wood extractives" that include pitch and volatile organic compounds.
To produce boards (oriented strand board, particle, veneer) composite wood products, and paper and pulp products, raw logs or wood fibrous material must be reduced to wood shavings, flakes, wafers, or fibers. These wood particulates are then further processed, either by bonding together with a suitable glue to make board products. However, the processing of logs into wood particulates, and thence into finished products, poses several challenges. Some of these arise from the nature of wood, namely, that it includes not only cellulosic fibers and lignin but also "wood extractives," as discussed above. These naturally-occurring wood extractives are found both in resin canals within the structure of the wood, as well as within the parenchyma cells of the wood. In general, the extractives may be divided into a higher molecular weight, higher boiling point fraction, commonly known as "pitch," and a lower molecular weight, lower boiling point fraction that falls within the definition of "volatile organic compounds." The United States Environmental Protection Agency (EPA) has determined that volatile organic compounds (VOCs) pose an environmental hazard when they are released into the atmosphere. These VOCs are defined in 40 C.F.R. Part 51(s) as "any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions." Typically, these are volatile, low molecular weight organic compounds. The EPA has promulgated regulations limiting the quantity of VOCs that a manufacturing facility may release into the atmosphere.
The release of VOCs into the atmosphere is a long-standing problem in the industry. Since VOCs occur naturally in timber, the processing of timber into wood particulates facilitates the migration or diffusion of VOCs to particulate surfaces from which the compounds vaporize into the surrounding atmosphere. As a practical matter, after the timer has been processed into particulates, the particulates undergo a drying stage. During drying, VOCs as well as moisture are driven off from the wood particulates into the environment. It is estimated that approximately one-third of the VOCs naturally-occurring in the wood is released into the atmosphere during the drying step. Moreover, in a subsequent step wherein the wood particulates are consolidated into a composite product, by applying heat and pressure, additional quantities of VOCs are released. Thus, once the pressure on the consolidate product is released, VOCs are emitted into the environment. It is estimated that another approximately one-third of the naturally-occurring VOCs of the wood is released into the environment in this step.
As illustrated in FIG. 1, some VOCs 2' are released into the atmosphere when logs 5' are processed in a debarking and flaking operation 10' into wood particulates. These particulates are typically stored in short-term storage bins 12', sized for ensuring continuous operation of the boardmaking process. Wood particulates from this storage undergo a drying step 14', releasing VOCs 15' into the environment. The dried wood particulates then enter a surge bin 16', for intermediate storage before further processing. In the board manufacturing step 20', the dried particulates from the surge bin are mixed with a resin (usually in liquid form) and the mixture is charged to formers or "felters" that form the particulates onto transport belts, screens, or caul plates, for example. The caul plates are loaded into a multi-opening or continuous press and then consolidated under heat and pressure into composite products. Upon pressure release on the press, VOCs 18' are emitted into the air. This constitutes about one-third of the VOCs originally in the wood. The composite product 22' includes the residual of about one-third of the total amount of extractives.
The EPA proposes limiting the amount of VOCs that any wood particulate processing facility releases into the atmosphere by regulations requiring permits. Since a wood particulate processing facility represents a significant capital investment, operators must take steps to limit VOC emissions while at the same time ensuring that processing equipment operate at or near full capacity for an adequate return on investment. To date, methods for limiting the quantity of VOC emissions have focused on enclosing the atmosphere surrounding any wood particulate process that may release VOCs and subjecting air within the enclosure to treatment for the removal of VOCs, before release of the air into the environment. These methods require expensive equipment including large hoods to enclose equipment, fans and ducts for transporting air containing VOCs, and incinerators for combusting VOCs in the air. The methods also have high combustion fuel costs.
There exists a need to reduce or eliminate the release into the environment of volatile organic compounds from processing operations that convert wood particulates into wood products. There also exists a need for wood products of enhanced strength, particularly wet strength, for use in structural applications. With available wood supplies declining, there also exists a need for composite wood products that provide the strength and stiffness that users have come to expect, but with reduced wood content. Such composites should have the added advantage of being of potentially lighter weight.